THERESA May told a Belfast audience that Northern Ireland got “great benefits” from being in the UK and that she would “always make the case for it”.

The Prime Minister was making a speech in the city at the end of a two day trip to Ulster where she also visited the border with the Republic for the first time since the EU referendum.

Her comments on the Union echoed many she has made to Scottish audiences, and provoked concern she was not acting as a neutral arbiter in terms of the peace process and as required under Good Friday Agreement. “Northern Ireland makes a major contribution to our Union, and it also derives great benefits from being an integral part of the UK,” she said.

“Every family and every business benefits from the strength and security that comes from being part of the world’s fifth largest economy. The rest of the UK is by far Northern Ireland’s biggest market, accounting for over half of its sales. I believe in the partnership of our four great nations in one proud Union and I want it to endure for generations to come. So a government I lead will never be neutral in our support for the Union. We will always make the case for it.”

During her speech she lashed out at internal Conservative critics of her plans for the Irish border, accusing them of being ready to “betray” the people of Northern Ireland and the Republic.

And she urged the EU to “evolve” its backstop on the border. In a sign of growing concern in Dublin about the prospect of a hard Brexit, Irish Taioseach Leo Varadkar suggested Ireland could close its airspace to UK planes if Britain seeks to ban EU ships from fishing in its waters.

May restated her opposition to the European Commission’s proposed “backstop” arrangement which would see Northern Ireland remain within the EU customs union and single market. This would involve the creation of a customs border within the UK, which was “something I will never accept and I believe no British Prime Minister could ever accept”, she said. Stephen Farry, of the Alliance party, hit out: “The backstop is...anything but a threat to the Good Friday Agreement, rather it recognises and protects Northern Ireland’s unique circumstances.”